If your telescope is less than a few years old, check the style of the back cell to see which version of the Feather Touch® SCT Micro Focuser you need: If your telescope has a flat back (left picture), you need the standard "C" Micro Focuser. If your telescope has a triangular pattern on the back (right picture), you will need the "CPC" version of the Micro Focuser. Known to Fit: Celestron 11inch SCT Schmidt-Cassegrain Draw Tube Travel Celestron NEXTSTAR 11 GPS